This invention relates to a device for converting heat energy to mechanical energy.
In more detail the invention relates to a device that utilizes a shape memory material to convert low temperature thermal energy to mechanical energy.
Sources of low-grade heat frequently exist at the site at which a demand exists for low-grade energy. For example, homes, farms, factories, office buildings, hotels, and the like frequently have a constant supply of hot water available, not all of which is utilized for its intended purpose. In addition, the waste heat of industrial, farm or home equipment may be economically utilized to heat water to a low temperature. Furthermore, it can confidently be predicted that low-grade heat in the form of hot water will in the near future be economically available almost everywhere from solar power. In general, this water is not hot enough to drive a turbine or use in any conventional type of energy-generating equipment.
Frequently, the energy demand may be for relatively low-power energy, either constant or constantly available even if not required constantly. Among the small-scale -- and yet important -- applications envisioned for heat engines working at low temperatures across a small .DELTA. T to produce a limited but significant amount of power are devices designed to orient solar apparatus, to regulate thermal heat control and as a thermal safety valve. Despite the limitations imposed on low-temperature heat engines, sources of low-grade heat are so wide spaced and available that utilization of a small fraction thereof could have a significant impact on the world energy supply.
Low temperature energy conversion utilizing heat engines based on the energetic shape recovery of a deformed thermoelastic material--called a shape memory material--have been extensively investigated. These "memory materials" are capable of undergoing a martensitic (diffusionless) transition with the ability of the alloys to undergo such a transition being temperature dependent. The temperature range at which this transition can occur is hereinafter called the critical temperature and is characteristic of the particular alloy under consideration. At temperatures below their critical temperatures, these alloys are highly ductile and may be plastically deformed. Annealed at a temperature above its critical temperature in a given shape (hereinafter designated the "heat-treated configuration") and deformed into a "deformed configuration" at a temperature below that critical temperature, such an alloy will revert back to its heat-treated configuration when heated to or above its critical temperature. The alloy will move in a direction opposite to the direction in which it had been deformed and in so doing will exert considerable mechanical force and can produce useful work.
These memory materials have been produced in shapes of wires, rods, foils, plates, tubes, springs, etc., and with wide ranges of temperature responsiveness which is a function of alloy composition and production history. For example, memory materials are available that possess critical temperatures in the range of from -150.degree. C. to +150.degree. C. The alloys of one such series, referred to as 55-Nitinol, have chemical compositions in the range from about 53 to about 57 weight percent nickel balance titanium, and are based on the intermetallic compound NiTi. Descriptions of these and other titanium-based memory materials are given in U.S. Pat. Nos. 3,174,851 (W. J. Buehler et al, "Nickel-Base Alloys," Mar. 23, 1965); 3,403,238 (W. J. Buehler et al, "Conversion of Heat Energy to Mechanical Energy," Sept. 24, 1968); and 3,558,369 (F. E. Wang et al, "Method of Treating Variable Transition Temperature Alloys," Jan. 26, 1971).
These alloys have found practical application in a number of different areas such as control devices and medical instruments. A wide variety of heat engines employing the alloys have also been proposed. A partial list of patents directed to such devices follows:
U.S. Pat. No. 3,913,326 Banks Oct. 21, 1975 PA1 U.S. Pat. No. 4,027,479 Cory May 6, 1976 PA1 U.S. Pat. No. 4,086,769 Smith May 2, 1978 PA1 U.S. Pat. No. 4,087,971 Hart May 9, 1978
The following publications also describe such devices.
Science Magazine, Vol. 191, Mar. 5, 1976 p.934-6
Intersociety Energy Conversion Conference 1975 record p.530-4
Scientific American, Apr. 1971, p. 118-122
Particularly pertinent to the present invention is U.S. Pat. No. 4,086,769. This patent relates to a compound memory engine incorporating a plurality of chambers each containing a work element of Nitinol in the shape of a coil spring. All of the Nitinol elements have been heat-treated in a contracted state to establish a memory position for the coil. One end of the Nitinol coils is attached to the back wall of a chamber and the other end to a plunger rod which in turn is connected to a crank shaft. When hot water is introduced into the chambers, the Nitinol coils return to their contracted memory position which provides the driving force to turn the crank shaft and stretch the other Nitinol elements in the system.
Although these and numerous other approaches have been proposed and utilized to provide thermally driven motive power sources, most prior art devices have been so mechanically complicated or grossly inefficient as to be impractical.
It is accordingly the object of the invention to provide a thermal engine which is mechanically simple and operationally feasible for certain applications.